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Generating Electricity With Jumping Water Droplets Or Humidity

Electricity can be generated via the action of tiny water droplets spontaneously jumping away from superhydrophobic surfaces, and can be harnessed to (potentially) power portable electronic devices, according to new research from MIT.

The researchers involved in the new work suggest that such a technology could allow for the creation of cellphones or tablets that are charged by nothing but the ambient humidity of the air. Alternately, they suggest that the technology could be used as a means of gathering drinking water.

The experimental chamber setup is seen from the front, with high speed camera looking into the chamber from the left. Photo Credit: Nenad Miljkovic and Daniel J. Preston

Notably, devices utilizing the new approach needn’t be complex, they can be designed to be quite simple, researcher Nenad Miljkovic argues. Perhaps as simple even as just a series of interleaved flat metal plates — maybe even super cheap aluminum.

Initial testing has so far focused mostly producing relatively small amounts of power (15 picowatts, or trillionths of a watt, per square centimeter of metal plate), but it will be easy to fine-tune the process to achieve at least 1 microwatt, or millionth of a watt, per square centimeter, according to the researchers. Which would put it on a similar footing to other methods of harvesting otherwise unused and near-ambient energy from the environment — such as via waste-heat harvesting, vibration harvesting, and micro air currents.

For example, Miljkovic has calculated that at 1 microwatt per square centimeter, a cube measuring about 50 centimeters on a side — about the size of a typical camping cooler — could be sufficient to fully charge a cellphone in about 12 hours. While that may seem slow, he says, people in remote areas may have few alternatives.

There are some constraints: Because the process relies on condensation, it requires a humid environment, as well as a source of temperatures colder than the surrounding air, such as a cave or river.

The system is based on Miljkovic and fellow researcher Evelyn Wang’s 2013 finding — in attempting to develop an improved heat-transfer surface to be used as a condenser in applications such as power plants — that droplets on a superhydrophobic surface convert surface energy to kinetic energy as they merge to form larger droplets. This sometimes causes the droplets to spontaneously jump away, enhancing heat transfer by 30% relative to other techniques. They later found that in that process, the jumping droplets gain a small electric charge — meaning that the jumping, and the accompanying transfer of heat, could be enhanced by a nearby metal plate whose opposite charge is attractive to the droplets.

This finding led to the more recent discovery that power can be generated simply by giving the second plate a hydrophilic surface. When the droplets “jump,” they transfer charge from one plate to the other. When connected via an external circuit, that charge difference can then be harnessed.

“In a practical device, two arrays of metal plates, like fins on a radiator, would be interleaved, so that they are very close but not touching. The system would operate passively, with no moving parts. For powering remote, automated environmental sensors, even a tiny amount of energy might be sufficient; any location where dew forms would be capable of producing power for a few hours in the morning.”

Images from a field emission scanning electron microscope show (left) an oxidized copper-oxide surface and (right) a copper-oxide surface with a 30 nanometer-thick hydrophobic coating. The inset images show a water droplet on the surface: At left, the droplet spreads out to wet the uncoated surface; at right, the droplet beads up on the hydrophobic surface, making very little contact. Image Credit: Nenad Miljkovic and Daniel J Preston

“The atmosphere is a huge source of power, and all you need is a temperature difference between the air and the device,” Miljkovic adds.

The new findings are detailed in a paper published in the journal Applied Physics Letters.

About the Author

James Ayre 's background is predominantly in geopolitics and history, but he has an obsessive interest in pretty much everything. After an early life spent in the Imperial Free City of Dortmund, James followed the river Ruhr to Cofbuokheim, where he attended the University of Astnide. And where he also briefly considered entering the coal mining business. He currently writes for a living, on a broad variety of subjects, ranging from science, to politics, to military history, to renewable energy. You can follow his work on Google+.

The list of potential energy harvesting sources includes the ubiquitous radio waves, more reliably available than humidity.

GCO: the idea is ultra-low-powered sensors/transmitters that don’t need batteries. A state-of-the art microcontroller like Freescale’s Kinetis KL03 (link), based on ARM’s tiniest M0+ CPU, fits in the dimple on a golf ball. I couldn’t find the typical power draw, but the sleep current is 1µA, which at 1.5V is 1.5µW. For energy harvesting, engineers need to get down from microwatts to nanowatts. They are working on it.

GCO

So that’s 10mW/m^2, several orders of magnitude less than solar cells.
I’m sure this will find interesting applications nonetheless, but powering consumer electronics like cellphones is unlikely to be one of them…

Wind Energy

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